1. Field of the Invention
This invention relates to a telephone system suitable for use as an internet telephone for exchanging audio data through the internet that is a worldwide computer network system.
2. Related Art
The internet is a worldwide computer network system connecting computer networks in corporations or universities beyond countries. Increasingly provided are various services using the internet, such as e-mail service, file transfer service, and information search service.
FIG. 1 schematically shows a general aspect of the internet. In FIG. 1, each of computer networks NET101, NET102, NET103, . . . has a plurality of terminals T which are connected together by LAN (Local Area Network) in form of Ethernet or a token ring.
These computer networks NET101, NET102, NET103, . . . are connected together through routers R101, R102, R103, . . . that route data from a computer network to another, depending on the destination of the data.
Computer networks NET101, NET102, NET103, . . . connected through the routers R101, R102, R103, . . . form a computer network system. The computer network system is called internet. The internet enables exchanges of data among computer networks NET101, NET102, NET103, . . .
The internet uses IP (Internet Protocol) as the protocol of its network layer. IP assigns an IP address to each terminal to identify a destination terminal of data. Each IP address is made up of four numerals each of which can be expressed by decimal 8 bits, such as 43.3.25.246.
As the internet is extended, the number of IP addresses possibly becomes insufficient. In some networks in which a large number of terminals are registered but only a small number of terminals are connected simultaneously, for example, it is possible to use a server on the network to allot currently available IP addresses to actually connected terminals alone in order to minimize the number of IP addresses used. In this manner, the network need not prepare IP addresses in the number corresponding to its terminals, but can effectively use a limited number of IP addresses.
The internet uses TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) as protocols of its transport layer. TCP permits communication after establishing a connection-type transmission connection, and deals with packet sequence control, re-transmission, flow control and congestion control. UDP is a connectionless-type protocol that is used in lieu of TCP in networks requiring real-time transmission. In digital audio transmission, for example, re-transmission is not requested even when a part of packets drops, but audio data is sent successively. In such audio transmission, UDP is used.
Thus, the internet basically uses TCP/IP protocol. That is, IP addresses are assigned to terminals of a computer network to identify individual terminals, and packets are transferred by TCP or UDP.
However, personal computers are not always connected by LAN, and there are some without IP addresses. Therefore, some individuals participating the internet use internet service providers. Through internet service providers, personal computers can be connected to computer networks and can participate the internet by, for example, PPP (Point to Point Protocol) or SLIP (Serial Line IP) through telephone lines.
FIG. 2 shows a construction of an internet service provider. The computer network NET151 of the internet service provider includes a server S151 and a router R151. The server S151 is connected to a public telephone line network TEL151 via modems M151, M152, M153, . . .
Terminals T151, T152, T153, . . . are those of individuals personally participating the internet.
Terminals T151, T152, T153, . . . are connected to the public telephone line-network TEL151 through modems (not shown). Individual terminals T151, T152, T153, . . . may be personal computers having serial ports.
For participation in the internet through an internet service provider, users previously make a contract with an internet service provider in most cases. When a contract is concluded between a user and an internet service provider, an account code and a password are sent to the user.
When an individual participates in the internet from one of the terminals T151, T152, T153, . . . the user dials into the internet service provider to call up the server S151 of the computer network NET151 of the provider. The server S151 responsively requests entry of the account code and the password for authentication whether the user is a contractor. When the server S151 authenticates that the entered account code and password are those of a contractor, it searches for an available IP address. If there is any IP address available, it temporarily assigns it to the terminal T151, T152, T153, or any other. Thus, the terminal obtaining the temporary IP address can connect to the internet.
In the above example, terminals are connected by PPP using telephone lines. However, ISDN (Integrated Service Digital Network) may be used alternatively. ISDN 64 includes three channels, namely, two B channels of 64 kbps and one D channel of 16 kbps. When ISDN is used, it can be used as a line of 64 kbps by sending IP packets on the B channels.
Internet telephones for effecting telephone communication using the internet are now being developed. Since the internet is basically free of charge, what is to be paid by the user for internet telephone communication through the internet is the charge based on the contract with the internet service provider and the charge for the call between the user and the internet service provider or the charge for the use of ISDN. Thus, users can enjoy long-distance telephone calls and international telephone calls very economically.
For a telephone call using the internet, audio data is transmitted in a compressed form. There are various systems for compression, and various sampling frequencies of audio data, such as 8 kHz, 10 kHz, 16 kHz, and so on, are used. As the sampling frequency becomes high, the data amount decreases, but the quality of sound becomes better. Systems using high compression rates are liable to deteriorate the quality of sound.
In a computer network system like the internet, the time for data transmission increases as the network becomes crowded. Therefore, while the network is crowded, it is necessary to use a lower sampling frequency for audio data and to employ a compression system with a higher compression rate in order to prevent that voice is interrupted due to a delay of transmission of the audio data. On the other hand, it is not wise to send audio data with a low sampling frequency and a high compression rate even when the network is not crowded because the quality of sound is not good.